This invention relates to a method and device for superimposing additional information on a video signal at a very low level which has as little effect as possible in a reproduced image.
Devices for recording digital information, such as digital VTR and MD (mini-disc) record players, are in widespread use, and DVD (digital video disc or digital versatile disk) devices with recording functions have also appeared.
In these digital information recorders, various additional information signals can be recorded together with the main video and audio signals or computer data, etc. In this case, the additional information signal is a digital signal recorded on an area distinct from digital information signal areas, such as for example the headers added to every block of data or TOC (Table of Contents) area.
In the case of a system where additional information is transmitted by superimposing on a conventional main video signal, therefore, the additional information signal is not superimposed directly on digital information signals such as headers, but is recorded on an indirect area.
The additional information signal may therefore easily be eliminated by filtering or tampering, so that necessary additional information signals can no longer be detected by recording and playback devices.
In particular, when control information and copyright information are added as additional information signals to prevent illegal duplication, their original purpose cannot be achieved due to the elimination of the additional information signal.
Moreover, if the additional information signal is added to an indirect area, only the main information signal will be obtained when the digital information signal is converted to an analog signal and the additional information signal will be lost.
This means that even if an anti-duplication control signal is added as an additional information signal to prevent illegal duplication, this strategy is completely ineffective when the signal is converted to an analog signal.
To solve this problem of the disappearance of the additional information signal when the signal is converted to an analog signal, the inventors already proposed a method wherein an additional information signal such as an anti-duplication signal is spectrally spread, and the spectrally spread additional information signal is superimposed on a video signal during digital or analog recording (U.S. patent application Ser. No. 08/75510).
In this method, spectral spreading is performed by, for example, generating a code of a PN (Pseudorandom Noise) sequence (hereafter, referred to as a PN code) at a sufficiently high rate and applying it to the additional information signal. An additional information signal such as a narrow-band, high level anti-duplication control signal is thereby converted to a wideband, very low level signal which has no effect on the video signal.
The additional information signal which has been spectrally spread in this way, i.e. the spectrally spread signal, is then superimposed on an analog video signal and recorded on a recording medium.
The video signal recorded on the recording medium may be either analog or digital.
In this method, the additional information signal, such as an anti-duplication control signal, is superimposed with the same timing and frequency as the video signal. It is therefore difficult for a person who wishes to perform illegal duplication to remove the superimposed anti-duplication control signal from the video signal.
However, the additional information signal such as a superimposed anti-duplication control signal can still be detected and used by performing reverse spreading.
Regarding ease of detection when the additional information is superimposed on a video signal as a spectrally spread signal, additional information bits can be spectrally spread by a spreading code synchronized with a sync signal of the video signal so as to generate a spectrally spread signal, and this spectrally spread signal then superimposed on the video signal.
If the spreading code is generated in synchronism with the video signal, it is merely required to generate, in synchronism with the sync signal of the video signal, a spreading code for performing reverse spreading when the additional information is to be detected. As it is simple to generate a reverse spreading code synchronized with the spreading code when the signal is superimposed, detection can be performed easily.
For example, the additional information may be spectrally spread using a spreading code having a field period synchronized with the vertical sync signal of the video signal.
From the viewpoint of ease of detecting the additional information when it comprises plural bits and making use of the characteristics of spectral spreading techniques, the bit information in each of the plural bits can be spectrally spread by different sequences of spreading codes. For example, CGMS (Copy Generation Management System) is a well-known anti-duplication control signal. In CGMS, the anti-duplication control signal comprises two bits of information: 00 which permits unrestricted duplication, 10 which permits only first generation copies, and 11 which prohibits duplication.
When these two bits of information are superimposed in synchronism with a video signal by the above-mentioned spectral spreading technique, the upper bit of information is spectrally spread by a first spreading code sequence, and the lower bit of information is spectrally spread by a second spreading code sequence, and the spectrally spread signals of the two sequences are superimposed at the same time on the video signal.
In spectral spreading technology, the two bits of additional information superimposed as described hereabove can be detected by performing reverse spreading of the first and second spreading code sequences which are synchronized with the image sync signal.
However, when plural bits are spectrally spread by different spreading code sequences and are all superimposed on the video signal, although they are each superimposed at a very low level, they together result in a part with a high level. In this part, therefore, the superimposed additional information may have an effect on the reproduced image leading to a deterioration of the S/N of the reproduced image.
Therefore, it was proposed that the spectrally spread signal obtained by spreading plural bits using different spreading code sequences should not be superimposed on all the areas of one screen, but in an area which does not interfere with observation of the screen such as the left or right edge.
However, when the spectrally spread signal obtained from the bits of additional information is concentrated in a fixed part of the screen, if the vertical sync signal of the video signal is changed in an effort to disable the anti-duplication control signal and the sync timing is offset, it may occur that the additional information signal can not be detected.
In particular, in the case of CGMS, the lower bit is [1] and if this lower bit is not detected, it may no longer be possible to prevent duplication.
In the case of CGMS, even if the upper bit is [1], it is not possible to distinguish whether duplication is prohibited or permitted, but if the lower bit is [1], it means that duplication is prohibited.
In other words in the case of CGMS, the lower bit has high priority, and if this high priority information bit is missing, effective anti-duplication is impossible.
This invention, which was conceived in view of the aforesaid problems, therefore aims to provide a method and device for superimposing important bits of additional information on a video signal with as little effect on the video signal as possible, and such that the additional information is not lost even when an attempt is made to illegally modify the video signal.
In the method and device for superimposing an additional information signal on a video signal according to this invention, spreading code sequences are generated corresponding to different predetermined parts of a video signal in synchronism with a sync signal of the video signal, bit information signals of the additional information signal comprising plural bits are spectrally spread by the spreading code sequences to generate spectrally spread bit information signals corresponding to the plural bits, and the spectrally spread bit information signals are superimposed on corresponding different predetermined parts of the video signal.
According to this invention, relatively high priority bit information signals of the spectrally spread additional information signal are superimposed on high priority parts of the video signal, so high priority information in the additional information can be protected.